Counties in California, Oregon, and Washington with more than 27 inches of precipitation a year had a significantly higher prevalence of autism than drier counties (P<0.01), reported Sean Nicholson, Ph.D., of Cornell University here, and colleagues in the November issue of the Archives of Pediatric and Adolescent Medicine.

The magnitude of the association in their observational study was substantial, the researchers said. They estimated that eliminating precipitation as a factor would cut the prevalence by 33% to 43%.

The findings suggested that bad weather might be an environmental trigger for autism among genetically vulnerable children, possibly because rainy weather forces children indoors, they said.

Indirect mechanisms could include increased television watching, reduced vitamin D levels, and elevated exposure to indoor chemicals, such as cleaning products or other pollutants, they hypothesized.

Or precipitation could have a more direct effect if it brings down chemicals from the upper atmosphere that act as an environmental trigger or if it increases use of pesticides, they added.

However, Noel S. Weiss, M.D., Dr.P.H., of the University of Washington in Seattle, urged caution in interpreting what he called tentative results.

In his accompanying editorial, Dr. Weiss said other explanations are possible, such as the considerable variation in criteria used to diagnose autism and the completeness of reporting.

"In Oregon and Washington, for example," he wrote, "could it be that state agencies in the western, rainy, relatively urbanized counties have enumerated a greater proportion of children with autism than their counterparts in the eastern, arid, relatively more rural counties?"

The findings are no cause for alarm, he emphasized, and should be used, if at all, primarily in stimulating further research for autism's etiologies.

Following release of CDC statistics suggesting northern states tended toward a higher prevalence of autism than southern states, the researchers examined annual precipitation as a possible environmental factor.

They studied autism prevalence in Oregon and Washington, which have a four-fold variation in precipitation between counties east and west of the Cascade Mountain range, and California, which has a relatively high variability in precipitation as well.

Information from the National Climatic Data Center allowed comparison of relative precipitation between counties, and county-level data were used for autism rates among children born in those counties between 1987 and 1999.

During these children's school years through the end of 2005, autism rates were much higher in the western counties of Oregon and Washington than in drier eastern counties, the researchers said.

Counties with more than 27 inches of precipitation annually tended to be the same ones with higher than average autism prevalence whereas those with less than 22 inches a year tended to have lower than median rates.

In California counties, precipitation variation was lower and the link between precipitation and autism was weaker.

Overall, higher mean annual precipitation from 1987 through 2001 was significantly associated with higher autism prevalence after adjustment for differences in population size, demographics, per capita income, state, and whether a California county had a regional developmental services center (correlation coefficient 0.0034, 95% confidence interval 0.0018 to 0.0050).

Likewise, the incidence rate ratio for diagnosis of autism among school-age children in 2005 was significantly higher for counties with more rain and snow (IRR 1.007, 95% CI 1.003 to 1.011).

The association with precipitation was also significant in a birth cohort of children younger than three -- "the time during which autism symptoms emerge and any putative, postnatal factor would be present"-- in Oregon and California counties with a regional center (coefficient 0.0079, 95% CI 0.0021 to 0.0137).

The researchers cautioned that their findings were not definitive and that further research would be needed to identify the trigger associated with precipitation.

The findings may have been biased by unmeasured factors such as whether wetter areas use broader diagnostic criteria for autism diagnosis or whether families that settle in these areas may be more prone to having autistic children, the researchers acknowledged.

But persistence of the results in county-level, fixed-effects regression analysis made it unlikely that these factors could explain the associations, they noted.

The study was supported by unrestricted research grants from Cornell University.

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